412 SHORT MEMOIRS ON METEOROLOGICAL SUBJECTS. 



increasing with the depth, but still, as is well known, always of slight 

 intensity; (the precipitation on the east side of the windrose, where the 

 polar current merges into the equatorial). 



At least, this much is clear, that the view now very prevalent that the 

 upper trade-wind or the SW. currents of our hemisphere bring a large 

 part of its aqueous vapor from its place of origin, the equatorial calm 

 zone, is not tenable. At the low temperature that it must have in the 

 high region above the trade- wind, it can, even if saturated, only contain 

 so little water, that, sinking down to the earth without the addition of 

 water from below, it can oidy arrive with a great relative dryness. 

 Thus, for instance, the observations of Piazzi Smyth on the Peak of 

 TeueriSe, at the lower limit of the return trades, at an altitude of 326+ 

 meters, gave the average tension of vapor for August and September 

 at 4 millimeters, therefore a dew-point of —2° C. This current could 

 therefore first produce a precipitation in regions whose temperature lay 

 below— 20C., and therefore it is impossible that our summer and 

 autumn rains come from this source. Karl v. Fritsch confirms the slight 

 amount of moisture in the anti-trades upon the Peak of Teneriffe, and 

 says that the higher SW. wind has a cooling, not a warming, effect^" 

 (probably on account of its greater violence). Muhry imagines the 

 water contained in the anti-trade to be in the solid form of the ice 

 spiculte of the cirrus clouds, and, indeed, the upper SW. current could 

 carry with it some water in solid form. But if the material for the 

 abundant precipitation of our SW. and W. winds comes in this manner 

 from the zone of calms, or from any part of the tropical zone, then the 

 upper west current above the zone of trade-winds must cover the sky 

 with a thick dark cirro-stratus layer, while to the contrary the zones of 

 trade-winds are the clearest regions of the earth. 



Therefore the rains of the anti-trades and of the west winds of higher 

 latitudes do not originate in the tropics, but the anti-trade receives its 

 moisture anew first from below, and can take up much water since it is 

 warmed by descending, and can again condense it so soon as it is cooled 

 in its horizontal progress or by ascent upon mountains. 



If no aqueous vapor could condense in ascending masses of air, then 

 must their ascensive power be rapidly paralyzed, since the cooling of the 

 ascending air is much more rapid than the usual upward diminution of 

 temperature. Therefore dry ascending air-currents could only under 

 rare, specially favorable circumstances attain to great altitudes. So soon, 

 however, as vapor is condensed, the temperature diminution of the 

 ascending air is so delayed that its ascensive power becomes very pow- 

 erful, and since thereby again the intensity of the precipitation increases, 

 the upward impulse remains in power until the aqueous vapor has nearl^^ 

 exhausted itself. Since now the diminishing pressure of the air partly 

 compensates the diminution of the quantity of vapor (see table, page 29), 

 therefore the buoyancy diminishes only slowly. The ascent of the damp 

 air is finely displayed in the rapid growth of the cumulus clouds, which 



